Wind turbine usually comprise at least the following principal components: a tower, a nacelle, a generator and a rotor comprising a hub and wind turbine blades. The nacelle is placed on top of the tower and the generator is situated inside the nacelle. The rotor is connected to the generator by means of a drive train.
In recent years, the construction of wind turbines has become more and more of a challenging task due to the general tendency to considerably increase sizes and weights of modern wind turbines. Mounting a wind turbine may usually include transporting wind turbine components to a site of construction, assembling a tower, lifting and mounting a nacelle with a generator to the tower, assembling a rotor on the ground, i.e. mounting wind turbine blades to the hub, and lifting and mounting the assembled rotor to the nacelle. For lifting these wind turbine components mobile cranes can be applied.
EP 2003333 A1 discloses one such method of mounting wind turbine components. According to this method, blades are mounted to a hub on the ground using a reach stacker and the assembled rotor is then lifted to a nacelle. However, such method has several drawbacks. Assembling the rotor on the ground can be difficult since it requires a large, clear and stable area in order to provide for suitable conditions for workers and the devices applied. In addition, lifting the assembled rotor to the nacelle is a complicated procedure since, besides an enormous size and weight of the rotor, it has to be rotated from a horizontal into a vertical position during lifting. A horizontal position of the rotor would mean that the rotor blades are orientated essentially horizontally, i.e. parallel to the whereas the vertical position is again defined by the essentially vertical orientation of the rotor blades.
An alternative method of mounting a wind turbine rotor to a nacelle is disclosed in EP 1925582 A1. This method includes mounting blades to a hub which has already been mounted to the nacelle before. The orientation of a blade is kept substantially horizontal when it is lifted off the ground and mounted to the hub through a corresponding empty blade hole on the hub. After mounting the first blade the hub is turned, so that a next blade, which again is lifted substantially horizontally, can be mounted to a next empty blade hole on the hub. The process is repeated until the last blade has been lifted and mounted to the hub. Apart from other drawbacks, this method has the disadvantage that it cannot be applied when it is not possible to turn the hub during the mounting process, e.g. when no or little electrical power is available during the installation to turn the hub.